We have been studying a post-translational modification of tubulin (the protein building block of microtubules) which we think may regulate microtubule assembly or function. This involves the reversible enzymatic addition of a tyrosine residue to the C-terminus of the alpha chain. We have found that membrane bound tubulin differs from cytoplasmic tubulin of brain in lacking any C-terminal tyrosine, and have tentatively identified changes in tyrosylation state that accompany the transitions from disassembled to assembled, and from cytoplasmic to mitotic microtubules, in cultured cells. Completely tyrosylated and detyrosylated tubulin have been used to compare many parameters of in vitro assembly. Tubulin tyrosylation has been found to occur in invertebrates, as well as mammals. In a second project we have identified vanadate as a specific inhibitor of ciliary dynein, and investigated its use as a probe for the possible involvement of dynein in cytoplasmic motility.